Pinocytosis and intracellular degradation of exogenous protein: modulation by amino acids

Intracellular degradation of exogenous (serum) proteins provides a source of amino acids for cellular protein synthesis. Pinocytosis serves as the mechanism for delivering exogenous protein to the lysosomes, the major site of intracellular degradation of exogenous protein. To determine whether the availability of extracellular free amino acids altered pinocytic function, we incubated monolayers of pulmonary alveolar macrophages with the fluid-phase marker, [14C]sucrose, and we dissected the pinocytic process by kinetic analysis. Additionally, intracellular degradation of endogenous and exogenous protein was monitored by measuring phenylalanine released from the cell monolayers in the presence of cycloheximide. Results revealed that in response to a subphysiological level of essential amino acids or to amino acid deprivation, (a) the rate of fluid-phase pinocytosis increased in such a manner as to preferentially increase both delivery to and size of an intracellular compartment believed to be the lysosomes, (b) the degradation of exogenously supplied albumin increased, and (c) the fraction of phenylalanine derived from degradation of exogenous albumin and reutilized for de novo protein synthesis increased. Thus, modulation of the pinosome-lysosome pathway may represent a homeostatic mechanism sensitive to the availability of extracellular free amino acids.     

Glutathione metabolism in normal and cystinotic fibroblasts

Intracellular concentrations of glutathione and activities of the enzymes gamma-glutamylcysteine synthetase, glutathione synthetase, and gamma-glutamyl transpeptidase were measured in confluent cultured human fibroblasts cell lines from 14 normal cell lines and four cystinotic cell lines. gamma-Glutamyl transpeptidase had a wide range of variability while the glutathione synthetic enzymes, gamma-glutamylcysteine synthetase and glutathione synthetase, had narrower variations and also exhibited no apparent relationship to glutathione content. No differences in the activities of these enzymes were found between normal and cystinotic cells in confluent cell cultures. The activities of the above enzymes and the cell number and content of glutathione, cystine, DNA, and total protein in two normal and two cystinotic fibroblast cell lines were measured during growth. The following growth-dependency patterns were observed: (1) gamma-glutamylcysteine synthetase activity increased markedly in lag and early log phases in both normal and cystinotic cells and decreased rapidly to low confluent levels thereafter. (2) gamma-Glutamyl transpeptidase showed the same wide range of activity noted at confluency but activities decreased in the log phase of growth, a pattern also seen in cystinotic cells. (3) Glutathione synthetase activity remained relatively constant during growth of normal cells but exhibited a peak of activity during lag and early growth of cystinotic cells. (4) Comparative glutathione levels of normal and cystinotic cells were not significantly different and exhibited similar fluctuations with time. (5) The cystine content of normal and cystinotic cells unexpectedly rose to high levels in the lag phase, then decreased to 0.1 nmol 1/2 cystine/mg protein in normal cells and to 0.3 to 1.2 nmol 1/2 cystine/mg protein in cystinotic cells during the log phase. As confluency was approached, normal cell cystine remained at low levels while cystinotic cell cystine rose to characteristically high levels of 50- to 100-fold greater than normal cells at late confluency. These studies extend our understanding of the regulation of glutathione and cystine content in cultured fibroblasts and suggest that glutathione content is closely controlled throughout the cell cycle in the face of varying activities of its anabolic and catabolic enzymes.     

Glutathione in fibroblasts from normal and cystinotic children

Intracellular levels of GSH and GSSG were compared in fibroblasts initiated from skin biopsies from normals and cystinotics. The concentration of GSH was essentially the same in both cell lines. However, the intracellular GSSG in cystinotic cells averaged less than half of that in normal cells. The ratio of intracellular GSH:GSSG was 10.6 in cells from cystinotics as compared with 4.7 in cells from normals. A significant distortion in the ratio of GSH:GSSG serves as another marker of the derangement of sulfur-amino acid metabolism in cystinosis. Whether this effect is primary or secondary remains to be elucidated.     

Structural proteins of polyoma virus: proteolytic degradation of virion proteins by exogenous and by virion-associated proteases.

A model has previously been proposed for the genetic relatedness of the structural proteins of polyoma virus, based upon similarities in the peptide maps of the major capsid protein VP1 with the virion proteins VP2 and VP3. Newer evidence suggests that this model is incorrect, and that protein VP1 is a product of one viral gene and that the multiple components of VP2 and VP3 are products of a second viral gene. Two-dimensional peptide maps of several preparations of polyoma purified separately from four separate infected-cell lysates has shown a variable content of VP1 peptides in proteins VP2 and VP3, with some preparations being free of detectable VP1 material in VP2 and VP3. An alternative explanation for the presence of VP1 peptides in the regions of VP2 and VP3 of some polyoma preparations involves the cleavage of proteins of polyoma virions during exposure to proteolytic enzymes in lysates of infected cells or to endogenous proteolytic activity of virions. Prolonged incubation of infected-cell lysates at 37 degrees C leads to an increase in the amount of 86,000-dalton dimer of VP1, a decrease in the relative amount of VP1, a decrease in or a loss of the lower band of VP2, and the appearance of a new major protein band of approximately 29,000 daltons. Two-dimensional peptide maps of the new 29,000-dalton protein show that it contains some VP1 peptides, indicating that this protein is derived from proteolytic cleavage of VP1. In addition, extensively purified polyoma virus contains a proteolytic activity that can be activated during disruption of the virus with 0.2 M Na2CO3-NaHCO3 (pH 10.6) in the presence of 5 X 10(-3) M dithiothreitol.     

Elevated temperature produces cystine depletion in cystinotic fibroblasts

Increasing the incubation temperature of cystinotic fibroblasts to 40 or 43 degrees C produces a 70-80% decrease in lysosomal cystine content within 24-48 h. This effect is probably mediated by an altered substrate affinity for another lysosomal transport protein.     

pH-profile of cystine and glutamate transport in normal and cystinotic human fibroblasts

In the human recessive condition cystinosis, cystine transport has been reported to be normal in the plasma membrane but defective in the lysosome membrane. A possible explanation is that the transport systems at the two cellular sites are identical and that the defect in cystinosis affects the porter's ability to operate at the low pH of the lysosome. To test this hypothesis the uptake of 3H-labelled cystine and glutamate by normal and cystinotic human skin fibroblasts has been measured in vitro at pH 5.8, 6.5, 7.0, 7.4 and 8.0. Uptake of glutamate was more rapid than that of cystine. Uptake of cystine increased with increasing pH, but uptake of glutamate showed no marked pH-dependence. Transport in cystinotic cells was similar to that in normal cells, and similarly affected by pH. This finding is incompatible with the hypothesis proposed above. It is concluded that the cystine porters of the plasma membrane and the lysosome membrane are probably genetically distinct.     

Decrease of intracellular cystine content in cystinotic fibroblasts by inhibitors of gamma-glutamyl transpeptidase

Cystine content of skin fibroblasts derived from patients with cystinosis was decreased by inhibitors of gamma-glutamyl transpeptidase, the initial enzyme in glutathione catabolism. The addition of maleate or the gamma-glutamyl hydrazone of alpha-ketobutyric acid to culture medium (1-20 mM) resulted in dose-dependent decreases of up to 55% on intracellular cystine content of cystinotic cells in 24 h. L-Serine in sodium borate buffer (40 mM each) produced similar results and further decreased cystine levels to 14% of cystinotic control values after 10 days incubation. Analysis of intracellular amino acids showed that, in general, other amino acids remained unchanged following serine-borate treatment. These results suggest that cystine storage in cystinotic tissues may be related to metabolism of glutathione.     

Proteasome-dependent degradation of oxidized proteins in MRC-5 fibroblasts

Fibroblasts were exposed to various concentrations of hydrogen peroxide and the removal of oxidized proteins was followed by determining protein-bound carbonyls. Fibroblasts are able to increase the turnover of metabolically radiolabeled proteins after treatment with hydrogen peroxide. It was demonstrated for the first time, that the increased protein turnover was accompanied by a removal of protein-bound carbonyl groups. The proteasome-specific inhibitor lactacystin was able to inhibit the elimination of protein-bound carbonyl groups. Therefore, the key role of the proteasome in the degradation of oxidized proteins in fibroblasts could be demonstrated.     

The degradation of endogenous and exogenous proteins in cultured smooth muscle cells

The pathways of degradation followed by endogenous proteins in cultured smooth muscle cells were compared with the well-characterized lysosomal pathway involved in the degradation of apolipoprotein B of endocytosed LDL. Under conditions in which lysosomal activity towards ¹²⁵I-labeled LDL was almost completely inhibited by chloroquine and/or ammonium chloride, the degradation of short-lived and abnormal proteins, assessed by the release of [³H]phenylalanine, was reduced by only 10–17%. The basal rate of degradation of long-lived proteins was reduced by about 30% by the same inhibitors while the accelerated proteolysis found under nutrient-poor conditions could be completely accounted for by the lysosomal system as defined by these lysosomotrophic agents. Temperature studies indicated differences between the mechanisms involved in the degradation of long-lived proteins (E_a = 18 kcal/mol) and short-lived proteins (E_a = 10 kcal/mol). Arrhenius plots for the degradation of endogenous proteins showed no transitions between 15 and 37°C in contrast to the breakdown of LDL which ceased below 20°C. The results indicate that the degradation of rapid-turnover proteins is largely extralysosomal and that a significant breakdown of long-lived proteins occurs also outside lysosomes.     

The degradation of normal and analogue-containing proteins in MRC-5 fibroblasts

Proteins containing the arginine analogue conavanine were degraded much more quickly in MRC-5 fibroblasts than those containing only normal amino acids. The degradation of both classes of protein could be well described by a pair of exponential curves, the first representing an early rapid degradation and the second, a slower phase. There were no general trends in the variation with passage number of the cell's ability to degrade either normal or analogue-containing proteins, as judged by the half-lives of proteins. But there was an increase in the proportion of labelled normal falling into the early rapid degradation phase as the cells senesced in culture     

Effects of weak bases on the degradation of endogenous and exogenous proteins by rat yolk sacs.

In rat yolk sacs incubated in vitro, the rates of degradation of endogenous [3H]leucine-labelled proteins and of pinocytically ingested 125I-labelled bovine serum albumin were both decreased in the presence of either ammonium, methylammonium or ethylammonium ions (0-20 mM) or much lower concentrations of chloroquine (0-500 microM). These effects were also accompanied by an inhibition of pinocytosis, as measured by the rate of uptake of 125I-labelled polyvinylpyrrolidone, and by a fall in the [ATP]/[ADP] ratio within the tissue. Re-incubation in inhibitor-free medium of yolk sacs previously exposed to a weak base restored pinocytic and proteolytic capacities, except for tissues exposed to chloroquine at concentrations above 0.1 mM (these appeared to be cytotoxic); an attendent rise in [ATP]/[ADP] ratios to near normal values was also observed. Weak bases, at concentrations that fully arrested the breakdown of 125I-labelled albumin, failed to inhibit by more than 45% the degradation of [3H]leucine-labelled endogenous proteins. Since 125I-labelled bovine serum albumin has been shown to be degraded entirely intralysosomally by yolk sacs, this suggests either that the hydrolysis of endogenous proteins is shared between lysosomes and some other site or that, unlike 125I-labelled albumin, some endogenous proteins can be degraded within lysosomes at abnormally high pH.     

Cystine accumulation in cystinotic fibroblasts from free and protein-linked cystine but not cysteine

The accumulation of cystine in cystinotic fibroblasts from free and protein-linked cystine has been investigated. Cystine is not accumulated from cysteine but is readily accumulated from cystine. The accumulation from free cystine does not occur as a result of pinocytosis or from the degradation of a rapidly metabolized protein pool. Further studies of the degradation of disulphide-containing proteins by these cells may aid understanding of the mechanisms of proteolysis.     

The effects of decreased growth temperature on the cystine content of cystinotic fibroblasts

Cystinotic fibroblasts transferred from 37 degrees C to 28 degrees C accumulated additional cystine over the period from 4 to 7 days of incubation at 28 degrees C, after which the additional cystine was lost; warming (to 37 degrees C) of cells with elevated cystine stores led to rapid cystine loss. These results, taken together with previously published data showing cystine release from cystinotic fibroblasts incubated at above-normal temperature, are interpreted as indicating the presence in the cystinotic fibroblast lysosome membrane of a cystine-porter whose efficacy is increased by an increase in membrane fluidity. This porter may be the residual activity of the cystine porter that is known to be deficient in cystinosis, or it may be a second as yet unrecognized porter. It is further proposed that this porter is responsible for the presumed efflux of cystine from cystinotic lysosomes.     

Microinjection of endogenous and exogenous proteins into primary cultures of rat hepatocytes and the degradation of the injected proteins

1. A method to microinject proteins into cells through packaging proteins to erythrocyte ghosts (erythrocyte-mediated microinjection) was modified partially in order to apply the method to primary cultures of rat hepatocytes. 2. Degradation of the microinjected proteins was examined employing the improved method. The mean half-life of the injected endogenous liver protein was 20 hr. The data suggested that the injected proteins are degraded through both lysosomal and non-lysosomal proteolytic pathways probably depending on their structure. 3. The present method to microinject exogenous proteins into primary cultures of rat hepatocytes can be employed usefully for the investigations of protein metabolism in liver.     

Accumulation of cystine from glutathione-cysteine mixed disulfide in cystinotic fibroblasts; blockade by an inhibitor of gamma-glutamyl transpeptidase

Cystinotic and normal skin fibroblasts in tissue culture were treated with varying concentrations of reduced glutathione, oxidized glutathione and glutathione-cysteine mixed disulfide, substrates of gamma-glutamyl transpeptidase, the catabolic enzyme of the gamma-glutamyl cycle. Cystine accumulated more rapidly and to a greater extent from the glutathione-cysteine mixed disulfide in cystinotic than in normal cells. Inhibition of gamma-glutamyl transpeptidase activity by serine in a borate buffer partially blocked this accumulation of cystine. Reduced glutathione and oxidized glutathione have lesser effects on cystine accumulation. Stored cystine in cystinotic tissues may derive in part from glutathione-cysteine mixed disulfide via transpeptidation.     

Binding and degradation of platelet thrombospondin by cultured fibroblasts

Thrombospondin was purified from human platelets and labeled with 125I, and its metabolism was quantified in cell cultures of human embryonic lung fibroblasts. 125I-Thrombospondin bound to the cell layer. The binding reached an apparent steady state within 45 min. Trichloroacetic acid-soluble radioactivity was detected in the medium after 30 min of incubation; the rate of degradation of 125I-thrombospondin was linear for several hours thereafter. Degradation of 125I-thrombospondin was saturable. The apparent Km and Vmax for degradation at 37 degrees C were 6 X 10(-8) M and 1.4 X 10(5) molecules per cell per minute, respectively. Degradation was inhibited by chloroquine or by lowering the temperature to 4 degrees C. Experiments in which cultures were incubated with thrombospondin for 45 min and then incubated in medium containing no thrombospondin revealed two fractions of bound thrombospondin. One fraction was localized by indirect immunofluorescence to punctate structures; these structures were lost coincident with the rapid degradation of 50-80% of bound 125I- thrombospondin. The second fraction was localized to a trypsin- sensitive, fibrillar, extracellular matrix. 125I-Thrombospondin in the matrix was slowly degraded over a period of hours. Binding of 125I- thrombospondin to the extracellular matrix was not saturable and indeed was enhanced at thrombospondin concentrations greater than 3 X 10(-8) M. The ability of 125I-thrombospondin to bind to extracellular matrix was diminished tenfold by limited proteolytic cleavage with trypsin. Degradation of trypsinized 125I-thrombospondin was also diminished, although to a lesser extent than matrix binding. Heparin inhibited both degradation and matrix binding. These results suggest that thrombospondin may play a transitory role in matrix formation and/or organization and that specific receptors on the cell surface are responsible for the selective removal of thrombospondin from the extracellular fluid and matrix.     

The ATP dependence of the degradation of short- and long-lived proteins in growing fibroblasts

To characterize the system(s) responsible for degradation of short-lived and long-lived proteins in mammalian cells, we compared the concentrations of ATP required for the degradation of these classes of proteins in growing hamster fibroblasts. By treating CHEF-18 cells with increasing concentrations of dinitrophenol and 2-deoxyglucose, it was possible to reduce their steady-state ATP content by different amounts (up to 98%). These treatments caused a rapid decrease in the degradation of both short- and long-lived proteins. Removal of the inhibitors led to a prompt restoration of ATP and proteolysis. As ATP content fell below normal levels (about 3.1 mM), rates of proteolysis decreased in a graded biphasic fashion. Reduction in ATP by up to 90% (as may occur in anoxia or injury) decreased proteolysis up to 50%; and with further loss of ATP, protein breakdown fell more sharply. Degradation of both classes of proteins was inhibited by 80% when ATP levels were reduced by 98%. The levels of ATP required for the breakdown of short- and long-lived proteins were indistinguishable. Protein synthesis was much more sensitive to a decrease in ATP content than protein breakdown and fell by 50% when ATP was reduced by only 15%. Chloroquine, an inhibitor of lysosome function, did not reduce the degradation of either class of proteins in growing cells, but it did inhibit the enhanced degradation of long-lived proteins upon removal of serum (in accord with previous studies). Thus, in growing fibroblasts, an ATP-dependent nonlysosomal process appears responsible for the hydrolysis of both short- and long-lived proteins.